FIGS. 4 and 5 are a cross sectional view and an exploded perspective view of a conventional panel switch 501, respectively. Stationary contacts 2 are provided on an upper surface of an electrically insulating substrate 1. Each of stationary contacts 2 includes an inner stationary contact 2A and an outer stationary contact 2B spaced by a distance between stationary contacts 2A and 2B. A resist layer 3 is provided on the upper surface of the insulating substrate 1 by printing and applying insulating material. The resist layer 3 has circular openings provided therein exposing stationary contacts 2, respectively. The openings are independently separated from each other one another and are not communicated with each other. Movable contacts 4 are made of elastic metallic material. Each contact 4 having a circular dome shape opening downward. The center of a lower surface of movable contact 4 is located above inner stationary contact 2A of stationary contact 2 by a distance. Movable contact 4 and stationary contact 2 provides a single switch.
Adhesive layer 6 is provided on a lower surface of cover sheet 1. Cover sheet 5 is bonded with the adhesive layer 6 onto the upper surface of the insulating substrate 1 so that movable contacts 4 are sandwiched between the cover sheet 5 and the insulating substrate 1.
FIG. 6 is a top view of the cover sheet 5. The adhesive layer 6 is formed by screen printing on the lower surface of the cover sheet 5. Circular adhesives 6A are provided on the lower surface of cover sheet 5 for holding the centers of movable contacts 4 to adhere onto the lower surface of the cover sheet 5, respectively. Air passages 7 where the adhesive layer 6 is not formed to expose the lower surface of cover sheet 5 are provided on the lower surface of the cover sheet 5. The air passage 7 includes ring portions 8 corresponding to the outlines of the movable contacts 4 and communicating portions 9 for communicating ring portions 8 adjacent to each other, thereby allowing movable contacts 4 to communicate with each other.
An operation of conventional panel switch 501 will be described below. Upon being pressed down at the center across the cover sheet 5, the movable contact 4 receives a pressing force. As the pressing force exceeds a predetermined level, the dome shape of the movable contact 4 elastically reversed with a light click feel. Then, the lower surface at the center of the movable contact 4 contacts the inner stationary contact 2A, thereby connecting electrically between the inner stationary contact 2A and the outer stationary contact 2B via the movable contact 4, thus turning on the switch 501.
When the pressing force applied to the cover sheet 5 is released, the dome shape of the movable contract 4 elastically returns back to its original shape which is upwardly convex with a light click feel. Consequently, the lower surface at the center of the movable contact 4 departs from the inner stationary contact 2A, as shown in FIG. 4, thus turning off the switch 501.
Since the cover sheet 4 covers the upper surface of the insulating substrate 1, the air trapped between the insulting substrate 1 and the lower concave surface of the movable contact 4 is compressed by the above elastic reversing of the movable contact 4. The compressed air may disturb the light click feel generated upon the elastic reversing of the dome shape of the movable contact 4. For compensation, the conventional panel switch 501 has the air passage 7 which is provided between the insulating substrate 1 and the cover sheet 5 and which does not have the adhesive layer 6. When the movable contact 4 is pressed down and elastically reversed, the air beneath the movable contact 4 flows through the air passage 7 and moves into another movable contact 4 which is not pressed down. This operation prevents the air from being compressed to disturb the light click feel, thereby allowing the movable contact 4 to maintain the click feel.
In order to increase the physical strength of the panel switch 501, the adhesive layer 6 is required to have a large boding strength to have the insulating substrate 1 adhere to the cover sheet 5. In the conventional panel switch 501, the adhesive layer 6 is provided on the lower surface of the cover sheet 5 by a screen printing technique enabling a pattern to be printed to form the air passages 7. The screen printing technique is however not favorable for increasing the thickness of the adhesive layer 6, accordingly preventing the adhesive layer from having a large bonding strength.